Solid-state NMR spectroscopy as a tool supporting optimization of MALDI-TOF MS analysis of polylactides

We report systematic structural studies of poly(l-lactide) (PLLA) employing matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) and solid-state NMR spectroscopy. (13)C cross polarization magic angle spinning (CP/MAS) NMR data for 1,8-dihydroxy-9-anthracenone (DT), 2,5-dihydroxybenzoic acid (DHB), 2-(4-hydroxyphenylazo)-benzoic acid (HABA), and trans-3-indoleacrylic acid (IAA), four matrices commonly used in MALDI-TOF analysis of polymers, were analyzed to test the influence of crystallization conditions (solvent, inorganic salt) on sample morphology. (13)C principal elements of chemical shift tensor (CST) and line-shape analyses were employed to study of the nature of hydrogen bonding and to evaluate the crystallinity and amorphicity of the pure polymer. NMR parameters for PLLA were compared with data for polylactide crystallized with the four matrices under different conditions with the addition of two inorganic salts as cationization agents. This study revealed that the semicrystalline structure of the polymer does not change when it is embedded in the matrix.     

95Mo NMR: a useful tool for structural studies in solution

Oxomolybdenum(VI) complexes containing diverse ligands from an electronic and topological point of view have been analysed by means of ⁹⁵Mo NMR in solution with the purpose of using this technique as a tool to study their coordination chemistry and reactivity. The relationship between the electronic density on the metal tuned by the electron‐donor ability of the coordinated ligands and the ⁹⁵Mo chemical shift has been proved for mono‐ and bimetallic complexes showing a hexa‐ or hepta‐coordination around the metal centre. The different origins of the signal broadening (associated either to the symmetry of the metallic polyhedron or to the presence of isomers or to the ligand de‐coordination) have been also considered to rationalise the obtained data. Copyright © 2009 John Wiley & Sons, Ltd.     

207Pb-1H two-dimensional NMR spectroscopy: a useful new tool for probing lead(II) coordination chemistry

Despite the fact that lead poisoning is the most common disease of environmental origin in the United States, the spectroscopic properties of aqueous Pb(II) coordination compounds have not been extensively investigated. Spectroscopic techniques that can be used to probe the fundamental coordination chemistry of Pb(II) will aid in both the development of water-soluble ligands that bind lead both tightly and selectively and the characterization of potential biological targets. Here, we report the preparation and characterization of a series of Pb(II) complexes of amido- derivatives of EDTA. The 207Pb chemical shift observed in these complexes (2441, 2189, and 1764 ppm for [Pb(EDTA)]2-, Pb(EDTA-N2), and [Pb(EDTA-N4)]2+, respectively) provides an extremely sensitive measure of the local environment and the charge on each complex. These shifts help to map out the lead chemical shift range that can be expected for biologically relevant sites. In addition, we report the first two-dimensional 207Pb-1H heteronuclear multiple-quantum correlation (HMQC) nuclear magnetic resonance spectra and demonstrate that this experiment can provide useful information about the lead coordination environment in aqueous Pb(II) complexes. Because this technique allows 207Pb-1H couplings through three bonds to be identified readily, 207Pb-1H NMR spectroscopy should prove useful for the investigation of Pb(II) in more complex systems (e.g., biological and environmental samples).     

Sequential multiphoton ionization spectroscopy: extension of a useful spectroscopic tool

An adaptation of multiphoton ionization spectroscopy is presented in which a single vibrational—rotational level of an excited electronic state is pumped using a dye laser, and a second, independently tunable dye laser beam induces multiphoton ionization from this excited level. Several advantages of this technique are demonstrated using molecular iodine.     

Solid-state NMR spectroscopy: an advancing tool to analyse the structure and properties of metal–organic frameworks

Metal–organic frameworks (MOFs) gain increasing interest due to their outstanding properties like extremely high porosity, structural variability, and various possibilities for functionalization. Their overall structure is usually determined by diffraction techniques. However, diffraction is often not sensitive for subtle local structural changes and ordering effects as well as dynamics and flexibility effects. Solid-state nuclear magnetic resonance (ssNMR) spectroscopy is sensitive for short range interactions and thus complementary to diffraction techniques. Novel methodical advances make ssNMR experiments increasingly suitable to tackle the above mentioned problems and challenges. NMR spectroscopy also allows study of host–guest interactions between the MOF lattice and adsorbed guest species. Understanding the underlying mechanisms and interactions is particularly important with respect to applications such as gas and liquid separation processes, gas storage, and others. Special in situ NMR experiments allow investigation of properties and functions of MOFs under controlled and application-relevant conditions. The present minireview explains the potential of various solid-state and in situ NMR techniques and illustrates their application to MOFs by highlighting selected examples from recent literature.

Metal–organic frameworks (MOFs) gain increasing interest due to their outstanding properties like extremely high porosity, structural variability, and various possibilities for functionalization.     

NMR spectroscopy as a tool to investigate the degradation of aromatic compounds by a Pseudomonas putida strain

Pseudomonas putida strain KT2442, harbouring the pWW0 TOL plasmid, was grown with a number of different homologous aromatic acids as carbon sources. Small samples of liquid culture supernatant were collected and directly analysed by 2D NMR spectroscopy. In all cases similar compounds with olefinic signals were observed to accumulate. To elucidate the structures of these compounds, 2D NMR experiments with 500 and 600 MHz spectrometers equipped with a CryoProbe (Bruker BioSpin) were performed on samples obtained from a culture growing on 4‐methylbenzoate and, for ¹³C spectroscopy, on ¹³C‐labelled 4‐methylbenzoate. In all cases a 1,2‐dihydroxycyclohexa‐3,5‐diene‐carboxylate derivative was identified. The use of this technique helped us to identify easily some metabolites that were released into the solution by bacteria and to follow their secretion as a function of time. The high sensitivity of the present approach allowed a clear and rapid acquisition of spectra, notwithstanding the low concentration of the compounds. The benefits of introducing the use of NMR cryoprobes to perform metabolic pathway studies is demonstrated. Copyright © 2003 John Wiley & Sons, Ltd.     

79Br NMR spectroscopy as a practical tool for kinetic analysis

⁷⁹Br NMR spectroscopy has been used to monitor a series of reactions in which the bromide ion is produced, including the Menschutkin reaction of pyridine with a range of substituted benzyl bromides and a Heck coupling process. In cases where the process could also be monitored using ¹H NMR spectroscopy, the kinetic analyses using heteronuclear magnetic resonance spectroscopy were shown to be completely consistent. Both the utility of the process in following reactions which may be difficult to analyse using other techniques and the practical limitations associated with solvent choice are discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Solid-state NMR spectroscopy applied to a chimeric potassium channel in lipid bilayers

We show that solid-state NMR can be used to investigate the structure and dynamics of a chimeric potassium channel, KcsA-Kv1.3, in lipid bilayers. Sequential resonance assignments were obtained using a combination of (15)N- (13)C and (13)C- (13)C correlation experiments conducted on fully labeled and reverse-labeled as well as C-terminally truncated samples. Comparison of our results with those from X-ray crystallography and solution-state NMR in micelles on the closely related KcsA K (+) channel provides insight into the mechanism of ion channel selectivity and underlines the important role of the lipid environment for membrane protein structure and function.     

Isotopically discriminated NMR spectroscopy: a tool for investigating complex protein interactions in vitro

A new NMR approach is presented for observing in vitro multicomponent protein-protein-ligand(s) interactions, which should help to understand how cellular networks of protein interactions operate on a molecular level and how they can be controlled with drugs. The method uniquely allows at least two polypeptide components of the mixture to be simultaneously closely monitored in a single sample, without increased signal overlap, and can be used to study complex (e.g., sequential, competitive, cooperative, allosteric, induced, etc.) binding events, witnessed by two polypeptides independently. One polypeptide is uniformly labeled with 15N and another with 15N and 13C. The 1H-15N correlation spectra are recorded for each of these molecules separately, discriminated on the basis of the type of 13C'/12C' atom attached to the amide group nitrogen. Any changes to the state of the two differently isotopically labeled molecules will be reported individually by fingerprint signals from amide groups, e.g., as unlabeled ligands are added. To our knowledge, no other technique currently exists which can monitor complex binding events in similar detail. The proposed method can be combined easily with traditional protein NMR techniques and incorporated in a variety of applications.     

Solid-state phosphorus-31 NMR spectroscopy of a multiple-spin system: an investigation of a rhodium-triphosphine complex

Phosphorus-31 NMR spectra of solid [tris(dimethylphenylphosphine)](2,5-norbornadiene) rhodium(I) hexafluorophosphate have been acquired at several applied magnetic field strengths. The phosphorus nuclei of the three phosphine ligands are spin-spin coupled to each other and to 103Rh, resulting in complex NMR spectra; however, the three phosphorus chemical shift (CS) tensors were determined through the analysis of NMR spectra of slow magic angle spinning and stationary samples. Spectra of spinning samples in rotational resonance and two-dimensional 31P NMR spectra were particularly useful for determining the magnitudes of the indirect spin-spin couplings, and to probe their signs. Despite being in similar environments, the three phosphorus nuclei of the phosphine ligands have distinct CS tensors. In particular, the spans of these tensors, delta11-delta33, range from 80 to 176 ppm. The phosphorus CS tensors have been assigned to specific sites determined by X-ray crystallography, based on a combination of the experimental results and the results of quantum chemical calculations of the phosphorus shielding and 2J(31P,31P) values. The effect of coordination of dimethylphenylphosphine with rhodium has been investigated by comparing calculated phosphorus CS tensors for the uncoordinated ligand with those obtained for the ligands in the complex.     

NMR shielding calculations across the periodic table: diamagnetic uranium compounds. 2. Ligand and metal NMR

In this and a previous article (J. Phys. Chem. A 2000, 104, 8244), the range of application for relativistic density functional theory (DFT) is extended to the calculation of nuclear magnetic resonance (NMR) shieldings and chemical shifts in diamagnetic actinide compounds. Two relativistic DFT methods are used, ZORA ("zeroth-order regular approximation") and the quasirelativistic (QR) method. In the given second paper, NMR shieldings and chemical shifts are calculated and discussed for a wide range of compounds. The molecules studied comprise uranyl complexes, [UO(2)L(n)](+/-)(q); UF(6); inorganic UF(6) derivatives, UF(6-n)Cl(n), n = 0-6; and organometallic UF(6) derivatives, UF(6-n)(OCH(3))(n), n = 0-5. Uranyl complexes include [UO(2)F(4)](2-), [UO(2)Cl(4)](2-), [UO(2)(OH)(4)](2-), [UO(2)(CO(3))(3)](4-), and [UO(2)(H(2)O)(5)](2+). For the ligand NMR, moderate (e.g., (19)F NMR chemical shifts in UF(6-n)Cl(n)) to excellent agreement [e.g., (19)F chemical shift tensor in UF(6) or (1)H NMR in UF(6-n)(OCH(3))(n)] has been found between theory and experiment. The methods have been used to calculate the experimentally unknown (235)U NMR chemical shifts. A large chemical shift range of at least 21,000 ppm has been predicted for the (235)U nucleus. ZORA spin-orbit appears to be the most accurate method for predicting actinide metal chemical shifts. Trends in the (235)U NMR chemical shifts of UF(6-n)L(n) molecules are analyzed and explained in terms of the calculated electronic structure. It is argued that the energy separation and interaction between occupied and virtual orbitals with f-character are the determining factors.     

Solid-state and high-resolution liquid 119Sn NMR spectroscopy of some monomeric, two-coordinate low-valent tin compounds: very large chemical shift anisotropies

High-resolution liquid- and solid-state 119Sn NMR spectroscopy was used to study the bonding environment in the series of monomeric, two-coordinate Sn(II) compounds of formula Sn(X)C6H3-2,6-Trip2 (X = Cl, Cr(eta 5-C5H5)(CO)3, t-Bu, Sn(Me)2C6H3-2,6-Trip2; Trip = C6H2-2,4,6-i-Pr3). The trends in the principal components of the chemical shift tensor extracted from the solid-state NMR data were consistent with the structures determined by X-ray crystallography. Furthermore, the spectra for the first three compounds displayed the largest 119Sn NMR chemical shift anisotropies (up to 3798 ppm) of any tin compound for which data are currently available. Relaxation time based calculations for the dimetallic compound 2,6-Trip2H3C6Sn-Sn(Me)2C6H3-2,6-Trip2 suggests that the chemical shift anisotropy for the two-coordinate tin center may be as much as ca. 7098 ppm, which is as broad as the 1 MHz bandwidth of the NMR spectrometer.     

Solid-state NMR spectroscopy of anionic framework aluminophosphates: a new method to determine the al/p ratio

A series of anionic framework aluminophosphates, with different Al/P ratios, have been investigated by various solid-state NMR techniques, including 27Al, 31P magic angle spinning (MAS), 27Al-->31P cross polarization (CP), 27Al{31P} rotational echo double resonance (REDOR), and 31P{27Al} transfer of population double resonance (TRAPDOR). Different Al coordinations (AlO4b, AlO5b, and AlO6b) and P coordinations (PO4b, PO3bOt, PO2bO2t, and PObO3t), where b represents bridging oxygens and t represents terminal oxygens, can be unambiguously determined based on the solid-state NMR spectroscopy. Furthermore, a new method to determine the Al/P ratio of open-framework aluminophosphates has been established, which is useful for the understanding of unknown aluminophosphate structures.     

Proton spin diffusion as a tool for characterizing polymer blends

Proton spin diffusion studies for characterizing minimum domain dimensions have been carried out on three blends: a 40/60 blend of nylon 6,6 with the aromatic rigid rod polymer poly(benzo[a,d]dithiazol-2,6-diyl-1,4-phenylene⁾, PBZT, a 47/53 blend of an amorphous nylon with PBZT, and a 50/50 blend of a polyetherimide (PEI) and a polybenzimidazole (PBI). Polarization gradients necessary for these experiments were generated using both chemical shift differences (via multiple pulse techniques) and linewidth differences. Polarization readout techniques included proton lineshape deconvolution, multiple pulse proton lineshapes and ¹³C CPMAS spectra utilizing short CP times. The two nylon/PBZT blends are expected to phase separate from thermodynamic arguments; however, kinetic considerations, more than thermodynamics, determine domain size. In the 40/60 blend, the minimum domain dimensions of each of the nylon and the PBZT phases were about 4 nm with some scattered larger crystals of nylon. In the 47/53 blend, mixing in some regions indicated domain dimensions similar to the 40/60 blend. In contrast to the 40/60 blend, however, the 47/53 blend was still far from internal spin polarization equilibrium after spin diffusion times of 140 ms. The implication is that the sample-average composition is not found over dimensions like 40 nm; the problem is that the possible morphological explanations are manifold. By investigating the proton rotating frame relaxation, T_1p, the possibility that some of the PBZT domains are isolated from the nylon, in a spin diffusion sense, was eliminated. It is more likely that about half of the nylon protons are isolated by spin diffusion from the PBZT protons on a 140 ms timescale. The PEI/PBI blend is a compatible blend of two aromatic polymers where mixing on a molecular scale is expected. We were interested in a measurement of the lower limit of domain size using proton spin diffusion. This lower limit turned out to be about 2.5 nm based on low temperature T_1p measurements as opposed to room temperature multiple pulse methods. The latter measurements monitored the disappearance of a polarization gradient between the PEI methyl protons and all the remaining protons. The superiority of the T_1p measurements over the multiple pulse method for establishing the smaller minimum domain dimension is not a general result and reasons are discussed. Finally, some general remarks about characterizing polymer blends by solid state NMR, particularly blends which have undergone spinodal phase separation, are included.     

Optically active ortho-metalated half-sandwich ruthenium complexes: solid-state NMR as a convenient tool to analyze mixtures of diastereomers

Bulk solid samples of various ratios of the cyclometalated arene ruthenium diastereomers (S)Ru- and (R)Ru-[(eta(6)-C6H6)Ru(C6H4-2-(R)-CH(Me)NMe2)PMe2Ph](+)PF6(-) (3a/3b), of which the configurational stability at the metal center has been established by classical solution techniques, have been analyzed by the 13C cross-polarization magic angle spinning (CP-MAS) and 31P MAS NMR. The spectra obtained allowed us to detect both isomers and to estimate their respective proportions by 31P spectra. This technique was applied to a bulk solid sample of the diastereomers (S)Ru- and (R)Ru-[(eta(6)-C6H6)Ru(C6H4-2-(R)-CH(Me)NMe2)NCMe](+)PF6(-) (1a/1b), which were shown to be configurationally labile by classical solution experiments. Detection of isomer 1a only in the resulting 13C CP-MAS NMR spectrum demonstrated that there has been epimerization of 1b to 1a during crystallization, thus confirming the configurational lability at the metal center.